I actually looked into the CFPS because I was curious; looks like there was some recent work on the subject looking into the question of measuring religious affiliation in China. The questionnaire doesn't ask about atheism (only religiously unaffiliated), and they noticed a drop in the total when the question was rephrased to be more broadly inclusive of traditional Chinese religions. Even with that modification, the majority of respondents (~70%) said they were unaffiliated, and I would consider that evidence of secularism; they most likely aren't religious/Christian per your original phrasing. If we assume that number is accurate it represents almost a billion people, definitely not "very few." The only number I could find on atheism that's recent was a Gallup poll, which suggested around 60% of people in China are "convinced atheists" but that appears to contradict actual scholarly research on the subject.
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I actually looked into the CFPS because I was curious; looks like there was some recent work on the subject looking into the question of measuring religious affiliation in China. The questionnaire doesn't ask about atheism (only religiously unaffiliated), and they noticed a drop in the total when the question was rephrased to be more broadly inclusive of traditional Chinese religions. Even with that modification, the majority of respondents (~70%) said they were unaffiliated, and I would consider that evidence of secularism; they most likely aren't religious/Christian per your original phrasing. If we assume that number is accurate it represents almost a billion people, definitely not "very few." The only number I could find on atheism that's recent was a Gallup poll, which suggested around 60% of people in China are "convinced atheists" but that appears to contradict actual scholarly research on the subject.